Personally I don't bother with split level. I just use the "Included disks" setting for each of my user shares. I have 2 disks filling with Movies, another with Backups, etc. When I need more space for a share, I either upsize one of the included disks, or add a disk and include it in the share.
The main purpose of split level is to keep things like music albums, or tv series, together so another disk won't have to spin up during playback. Just keep different things in different shares and include specific disks for each.